There is estimated to be over 5 x 1022g
of carbon on earth. One tonne is 106g, so this is 5 x 1016
tonnes, or to put it another way:

50,000,000,000,000,000 tonnes
of carbon on earth

About 99% of this is locked away in sedimentary
rocks such as limestone and chalk (both of these are made of calcium carbonate
CaCO3), the remainder can pass through the various carbon sinks
and reservoirs as part of the carbon cycle and so is in some manner "available".
This remaining carbon is distributed between a number of reservoirs, some
living - for instance you represent a carbon reservoir approximately equivalent
in size to a small bag of barbeque charcoal. Some is atmospheric, some
is dissolved
in water and some is buried beneath the earth.

Carbon reservoir

Mass in tonnes of
carbon x 109
(billions of tonnes of carbon)

% of total

Atmosphere

740.0

1.43

Land plants

550.0

1.06

Land animals

0.5

0.001

Marine plants (algae)

1.5

0.003

Marine animals

1.5

0.003

Dead organic matter (in soil and peat)

1600.0

3.08

Coal

4500.0

8.67

Oil and gas

500.0

0.96

Dissolved in the oceans

38 000.0

73.2

Marine sediments

6000.0

11.6

Totals

51 893.5

100.007

This distribution is often rather surprising
at first sight, notably because there is so much carbon dioxide contained
dissolved in the oceans and relatively little is contained in the atmosphere.
The atmosphere however is exceptionally important in that it is the transfer
medium between the other sinks and while relatively little stays in the
atmosphere a great deal passes through it.

The oceans are by far the largest sink of
carbon as dissolved carbon dioxide. This is medium-term storage and is in
a balance with atmospheric carbon dioxide, if atmospheric carbon dioxide
is removed, it will replaced by some that was dissolved in the oceans, if
large amounts of carbon dioxide are released into the atmosphere, much of
it will be absorbed by the oceans. The vast majority of carbon dioxide released
since the start of the industrial revolution is now held dissolved in the
worlds oceans if this was not the case, the atmospheric levels would have
risen far more than they even have.

It is difficult to appreciate numbers like this in
a relative manner by seeing them as a list,so here is a graphical way of
showing the data: